It is well-known for foodstuffs and beverages, such as preserves, pickles, baby food, fruit juice and the like, to be supplied in vacuum-sealed containers, in order to extend the shelf-life of the product. Typically, a partial vacuum is formed in the space, known as the headspace, between the surface of the product and the closure, or lid, of the container, during the filling and capping process. For example, the partial vacuum in a 70 mm diameter container may be between 18 and 24 inches of mercury (from 0.61 to 0.81 bar).
The internal diameter of jars vary in size due to the manufacturing process. Taking the smallest possible jar diameter and lowest vacuum gives a lower vacuum load of 175N (Newtons). Taking the largest possible jar diameter and highest vacuum gives an upper vacuum load of 240N. Hence, the strength requirement of each lug is at least 60N.
The majority of vacuum-sealed containers are threaded, the associated closure having lugs designed to co-operate with the thread, so that the closure can be twisted off the container during opening. Opening requires that two forces be overcome, namely that caused by the partial vacuum and that caused by friction between the screw threads. Typically, a soft sealing compound is applied around the underside of the closure to help maintain a tight seal between the container body and the closure.
For a 70 mm diameter container, the typical torque required to overcome the force of the vacuum seal is around 25 inch pounds (inlbs), while the torque required to overcome the frictional force of the screw thread is around 12 inch pounds (inlbs). It can therefore be difficult to open vacuum-sealed containers, particularly for consumers who are unable to apply the necessary force, such as the elderly. A study carried out in the UK showed that 33% of women over 55 are unable to open 50% of jars that they purchase due to the excessive torque.
A consequential problem with threaded containers is that the lugs of the closure may not be strong enough to cope with the torque required to open the container and may be bent in the process. This may cause the closure to simply spin, rather than unscrewing. The tendency for “spinners” to occur is accentuated by several factors, one being the “earing” of the steel. When a “spinner” occurs, the panel of the closure is not lifted, and the vacuum seal may remain intact. This can result in the container becoming impossible to open. Furthermore, even if the vacuum seal is broken, it may not be possible to reclose the container due to the deformation of the lugs. “Spinning” leads to customer dissatisfaction and potential wastage. Increased lug strength is often viewed as a solution to the problem of “spinning”. However, this can result in increased manufacturing costs due to increased use of metal.
For example, capping trials have been carried out on a high speed in-line capping machine. For this, 70 mm deep closures were manufactured with 4 lugs in a range of thicknesses. The vacuum was set to the top of the process window and glass jars were selected with the minimum diametral tolerance, thus they were as small as possible. The glass jar finish varies considerably in size due to the manufacturing process, a typical tolerance being +/−0.4 mm on diameter in the thread region.
For 0.17 mm thick steel closures the ‘spinning’ failure rate was measured at around 2.5%. When the gauge was increased to 0.18 mm the trials gave a 0.5% failure rate. Using a gauge of 0.19 mm and harder temper steel gave a 0% failure rate. The “spinner” issue was therefore solved but at the expense of producing thicker closures in a non-standard temper.
Easy close containers are known in the prior art. In particular, vacuum-sealed containers provided with venting features, such as channels or notches, are known. EP2662296A1 discloses a non-threaded, vacuum-sealed container which is provided with a venting feature on the container rim, while U.S. Pat. No. 7,861,874B2 discloses a threaded, re-closeable container having a lug configuration which includes a controlled venting path.